1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a lithium secondary battery having an improved high-rate discharging efficiency produced by varying the thickness ratio of cathode sheets fixed to both surfaces of a cathode current collector.
2. Description of the Related Art
Lithium secondary batteries are classified as lithium metal batteries, lithium ion batteries and lithium polymer batteries according to the type of lithium used. A lithium polymer battery is a battery in which lithium ions move between a cathode made of lithium oxide and an anode made of a carbon material during charging/discharging to generate an electromotive force. The lithium polymer battery has an advantage in that it is explosion-free due to the use of a solid polymer electrolyte having an ion conductivity higher than a liquid electrolyte. Also, unlike other kinds of batteries, the lithium polymer battery can overcome the problem of battery performance deteriorating due to byproducts generated during charging and/or discharging. Furthermore, due to its flexibility, much attention has recently been paid to lithium polymer batteries as next generation batteries.
Various types of lithium polymer batteries can be assembled according to the fabrication method of cathode and anode plates. A bi-cell structure is most widely employed. As shown in FIG. 1, a conventional bi-cell assembled battery is constructed such that a cathode plate 11 is fixed to both sides of an anode plate, 12, respectively, and a separator 13 is interposed between the cathode plate 11 and the anode plate 12 for insulating both plates 11 and 12 from each other.
Also, the cathode plate 11 includes a cathode current collector 14 made of a thin aluminum foil film, and a cathode sheet 15 fixed to at least one surface of the cathode current collector 14. The cathode sheet 15 is fabricated by adding a conductive material, a plasticizer and a binder to a cathode active material as a main component, such as lithium oxide.
The anode plate 12 includes an anode current collector 16 made of a thin aluminum foil film, and an anode sheet 17 fixed to at least one surface of the anode current collector 16. The anode sheet 17 is fabricated by adding a plasticizer, a conductive material and the like to an anode active material as a main component, such as a carbon material.
A plurality of openings 14a and 16a are formed in the cathode and anode current collectors 14 and 16, respectively.
The battery having the above-described configuration is fabricated as follows.
First, primary lamination is performed such that slurry-type source materials of the cathode and anode sheets 15 and 17 are fixed to both surfaces of the cathode and anode current collectors 14 and 16, thereby fabricating the cathode plate 11 and the anode plate 12.
Then, secondary lamination is performed such that the separator 13 is interposed between the cathode plate 11 and the anode plate 12, and predetermined amounts of heat and pressure are applied to the cathode plate 11 and the anode plate 12 with the separator 13 interposed therebetween to fuse them.
Subsequently, plasticizer components contained in the cathode and anode sheets 15 and 17 are extracted and an electrolyte material is impregnated in the space produced by extracting the plasticizer material, thereby forming a battery cell.
Next, the battery cell is inserted into a case having a space for accommodating the battery cell, and the case is hermetically sealed, thereby completing a battery.
The charging and discharging mechanism of a battery will now be described briefly.
Charging refers to a reaction in which lithium ions contained in the lithium oxide of the cathode plate 11 move to the anode plate 12 to then be intercalated between carbon molecules of the anode plate 12. Conversely, discharging refers to a reaction occurring when lithium ions intercalated between carbon molecules of the anode plate 12 move back to the cathode plate 11. Likewise, as lithium ions reciprocate between the cathode and anode plates 11 and 12, an electromotive force is generated.
In this case since the electrical capacity of a battery is generally proportional to the amount of the lithium oxide, which is a cathode active material contained in the cathode sheet 15, in order to increase the capacity of the battery, it is necessary to make the cathode sheet 15 thick.
However, if the cathode sheet 15 is excessively thick, the distance between the anode plate 12 and the cathode current collector 14 stacked to have the separator 13 interposed therebetween increases, causing a sharp decrease in the high-rate discharging efficiency. Thus, it is necessary to change the structure of the cathode plate 11 so as to improve the high-rate discharging efficiency, without changing the overall thickness of the cathode sheet 15.
To solve the above problems, it is an objective of the present invention to provide an lithium secondary battery having an improved high-rate discharging efficiency by varying the thickness ratio of cathode sheets fixed to both surfaces of a cathode current collector.
Accordingly, to achieve the above objective, there is provided a lithium secondary battery having a bi-cell type battery cell consisting of an anode plate, a separator fixed to both surfaces of the anode plate and a cathode plate fixed to either outer surface of the separator, wherein the cathode plate includes a cathode current collector, a front cathode sheet fixed to one surface of the cathode current collector, which is adjacent to the anode plate, and a rear cathode sheet fixed to the other surface of the cathode current collector and having a thickness different from that of the front cathode sheet.
Also, the thickness ratios of the front and rear cathode sheets is preferably in the range of 5:1 to 1:1.
Further, the front and rear cathode sheets may include one selected from the group consisting of lithium-based oxide, nickel-based oxide and manganese-based oxide, as a main component.
According to another aspect of the present invention, there is provided a lithium secondary battery including an anode plate having an anode current collector and an anode sheet fixed to at least one surface of the anode current collector, a cathode plate having a cathode current collector and a front cathode sheet fixed to one surface of the cathode current collector, which is adjacent to the anode plate, and a rear cathode sheet fixed to the other surface of the cathode current collector and having a thickness different from that of the front cathode sheet, and a separator interposed between the anode plate and the cathode plate.
Also, the thickness ratios of the front and rear cathode sheets is preferably in the range of 5:1 to 1:1.